1. Field of the Invention
The present invention relates to a control apparatus for an electric power steering system which assists steering operation by a vehicle driver by means of an electric motor.
2. Description of Related Art
In an electric power steering system which assists steering operation by a vehicle driver by means of an electric motor, an EPSECU (Electric Power Steering Electronic Control Unit) calculates an assitive steering torque on the basis of steering torque caused by torque applied to a steering wheel by a vehicle driver (referred to as steering wheel torque hereinafter) and various sensor signals including a steering wheel signal and a vehicle speed signal, and drives an electric motor in accordance with the calculation result in order to generate an appropriate assistive steering force depending on steering operation by the vehicle driver.
The EPSECU drives the motor such that the assistive steering force increases as the steering torque increases as basic control. According to such basic control, the vehicle driver can turn the steering wheel lightly, because when the vehicle driver turns the steering wheel, the assistive steering force depending on the steering torque at this time is generated.
However, if the basic control alone which determines the assistive steering force in accordance with the steering torque is performed, vehicle convergence is degraded due to road surface reaction force when the steering wheel is returned. That is, when a vehicle is running with its steering wheel being turned, there occurs torque (SAT; Self Aligning Torque) which acts to move the tire wheels of the vehicle to move to the straight forward direction of the vehicle. Accordingly, the steering wheel is moved back strongly, as a result of which the vehicle convergence is degraded.
To remove such problem, it is known to provide the electric power steering system with a damping control function which controls the rotation of the motor such that the motor is more suppressed from rotating when the motor speed (that is, the turning speed of the steering wheel) is higher. The damping control is control to alter, depending on the motor speed, the characteristic of the basic control which determines the assistive steering force in accordance with the steering torque. According to this damping control, the returning operation of the steering wheel involves a feeling of stickiness, and the vehicle convergence is improved, since the steering wheel returns slowly even when the road surface reaction force affects the steering wheel to move rapidly.
FIG. 4 is a diagram showing a conventional electric power steering system 100 having the damping control function. The electric power steering system 100 assists vehicle driver's operation of a steering wheel 2 by means of an electric motor 6. The steering wheel 2 is fixed to one end of a steering shaft 3 the other end of which is connected with one end of a torque sensor 4. The torque sensor 4 is connected to an intermediate shaft 5 at the other end thereof.
The torque sensor 4 is a sensor for measuring steering torque. In more detail, the torque sensor 4 includes a torsion bar coupling the steering shaft 3 to the intermediate shaft 5, and measures torque applied to the torsion bar on the basis of the torsion angle of the torsion bar.
The motor 6, which is for assisting the steering force of the steering wheel 2, is provided with a worm gear at the distal end of its rotation shaft, the worm gear being engaged with a worm wheel provided in the intermediate shaft 5 so that the rotation of the motor 6 is transmitted to the intermediate shaft 5. Conversely, when the intermediate shaft 5 is rotated by operation of the steering wheel 2 or reaction force from a road surface 12, the rotation of the intermediate shaft 5 is transmitted to the motor 6 to cause the motor 6 to rotate.
The motor 6 is a brushless DC motor having a rotation sensor disposed thereinside so as to be capable of outputting a signal indicative of the rotational speed (rotational angular velocity) of the motor 6.
The other end of the intermediate shaft 5 opposite to the one end to which the torque sensor 4 is fitted is connected to a steering gear box 7. The steering gear box 7 constitutes a gear mechanism including a rack and a pinion gear (not shown). The teeth of the rack are engaged with the pinion gear provided in the other end of the intermediate shaft 5. Accordingly, when the vehicle driver turns the steering wheel 2, since the intermediate shaft 5 rotates and accordingly the pinion gear rotates, the rack moves rightward or leftward. The rack is fitted with a tie rod 8 at each end thereof. The tie rods 8 move leftward or rightward together with the rack. As a result, since each of the tie rods 8 pulls or pushes a knuckle arm 9 disposed ahead of the tie rod, the direction of the tire 10 can be changed.
A vehicle sensor 11 is mounted on a predetermined portion of the vehicle to measure the vehicle speed. In the above described structure, when the vehicle driver turns the steering wheel 2, the rotation of the steering wheel 2 is transmitted to the steering gear box 7 through the steering shaft 3, torque sensor 4; and intermediate shaft 5. Further, the rotation of the intermediate shaft 5 is converted into side-to-side motion of the tie rods to steer the tires 10 on the left or right sides.
The EPSECU 110, which is powered by the electricity from a vehicle battery (not shown), calculates an assistive steering force on the basis of the steering torque measured by the torque sensor 4, the motor speed of the motor 6, and the vehicle speed measured by the vehicle speed sensor 11. The motor 6 is driven in accordance with the calculation result to appropriately control the assist amount for the force to move the steering wheel 2 (and accordingly the force to steer the tires)
In more detail, the EPSECU 110 is constituted of a basic assist amount calculating section 111 to calculate a basic assist amount, a damping control section 114 to calculate a damping compensation amount, an adding section 113 which adds the basic assist amount and the damping compensation amount to calculate an assist command value, and a motor drive circuit 112 to drive the motor 6 in accordance with the assist command value received from the adding section 113.
The basic assist amount calculating section 111 calculates the basic assist amount on the basis of the steering torque measured by the torque sensor 4 and the vehicle speed measured by the vehicle speed sensor 11. In more detail, the basic assist amount calculating section 111 calculates, by use of a steering torque-basic assist amount map, for example, the basic assist amount such that as the steering torque increases, the basic assist amount increases (that is, the torque to assist the steering wheel 2 to move increases), and as the vehicle speed increase, the basic assist amount decreases.
The damping control section 114 calculates the damping compensation amount on the basis of the motor speed to compensate the basic assist amount calculated by the basic assist amount calculating section 111. In more detail, the damping control section 114 calculates the damping compensation amount such that as the motor speed increases (that is, as the steering wheel 2 movers more rapidly), the motor speed is more suppressed (that is, the movement of the steering wheel 2 is more suppressed). Since various methods for performing the damping control in the electrical power steering system have been proposed and put to practical use, explanation of such methods is omitted here.
The motor drive circuit 112 supplies the motor 6 with a current in accordance with the assist command value obtained by adding the basic assist amount and the damping correction amount, in order to drive the motor 6. The assist command value inputted to the motor drive circuit 112 is the basic assist amount which has been compensated by the damping compensation amount. Accordingly, when the vehicle driver operates the steering wheel 2 to move more rapidly, or when the road surface reaction force causes the tires 10 to return to the straight forward direction more rapidly (that is, the steering wheel 2 is caused to move back to its neutral position more rapidly), the movement of the steering wheel 2 is more suppressed.
Other than the above, the EPSECU 110 includes various functional blocks such as a phase compensation section to improve the stability of the basic assist amount, a feedforward control section to improve response to variation of the steering torque, and a feedback control section to determine the current command value to be given to the motor drive circuit 112 by means of feedback control (PI control, for example) on the deviation between the assist command value and the actual current of the motor 6. However, they are omitted from FIG. 4 for ease of explanation.
The electric power steering system 100 having the above described structure is capable of giving a feeling of stickiness to rapid movement of the steering wheel 2 because of the provision of the damping control function. Accordingly, the electric power steering system 100 can improve the returning characteristic (returning feeling) of the steering wheel 2, and accordingly improve the vehicle convergence compared to other conventional electric power steering systems not having the damping control function.
However, on the other hand, such a damping control has a disadvantage that it may disturb the vehicle driver's operation of the steering wheel 2.
That is, since the damping control operates to suppress the movement of the steering wheel 2 when it is caused to move rapidly, the steering wheel 2 is suppressed from moving back rapidly when the steering wheel 2 is returned. However, the steering wheel 2 moves only slowly also when the vehicle driver tries deliberately to turn the steering wheel 2 sharply. That is, in this case, vehicle driver's operation of the steering wheel 2 is disturbed.
This is explained in further detail with reference to FIGS. 5A and 5B. FIG. 5A is a Bode diagram showing the frequency characteristic of the motor speed with respect to steering wheel torque, and FIG. 5B is a Bode diagram showing the frequency characteristic of the motor speed with respect to road surface reaction force of the electric power steering system 100 shown in FIG. 4.
As shown in FIG. 5B, the motor speed with respect to the road surface reaction force in the case where the damping control is provided is lower (especially in the range circled by the dotted line) than that in the case where the damping control is not provided and the motor 6 is driven only in accordance with the basic assist amount. That is, the motor speed is suppressed in the presence of road surface reaction force by the damping control. According to the above characteristic, the steering wheel 2 is suppressed from returning rapidly due to road surface reaction force, to thereby improve the vehicle convergence.
As shown in FIG. 5A, also the motor speed with respect to the steering wheel torque in the case where the damping control is provided is lower (especially in the range circled by the dotted line) than that in the case where the damping control is not provided and the motor 6 is driven only in accordance with the basic assist amount. That is, the motor speed is suppressed also when the vehicle driver turns the steering wheel 2. According to the above characteristic, the movement of the steering wheel 2 is more suppressed when the vehicle driver operates the steering wheel 2 to steer more sharply.
The foregoing patent document 1 discloses reducing the feeling of the steering wheel being forcibly moved back to the neutral position when the steering wheel is returned, while maintaining the feeling of the steering wheel being smoothly turned from the neutral position by making improvement to the damping control function.
In short, the technique disclosed in the patent document 1 is to adjust the damping compensation amount in accordance with the road surface reaction force (may be referred to as SAT hereinafter), and a signal indicative of distinction between turning and returning of the steering wheel. That is, the damping compensation amount is varied depending on the estimated value of the SAT such that the damping compensation amount increases to decrease the motor speed as the SAT increases. According to this technique, when the SAT is small, for example, when the steering wheel starts to be turned, since the damping compensation amount is small, the vehicle driver's operation of the steering wheel is less disturbed.
However, although the vehicle driver's operation of the steering wheel is less disturbed when the steering wheel starts to be turned, since the SAT occurs inevitably when the steering wheel is turned, the damping control operates more or less depending on the estimated value of the SAT. Accordingly, when the vehicle driver starts to turn the steering wheel, the damping control operates more or less to disturb the operation of the vehicle driver.
The SAT increases as the vehicle driver steers more rapidly. Accordingly, when the vehicle driver turns the steering wheel widely, since the estimated value of the SAT becomes large, the movement of the steering wheel (the rotation of the motor) is disturbed to a large extent.
Hence, even according to the technique described in patent document 1, when the steering wheel is turned, the damping compensation amount is calculated to a value corresponding to the estimated value of the SAT although it may be small, as a result of which the driver's operation is disturbed.
In addition, the technique described in patent document 1 needs to estimate the SAT. Since the control accuracy of the system depends on the accuracy of estimating the SAT, it is necessary to estimate the SAT with a sufficiently high degree of accuracy to enable the system to operate reliably. However, estimating the SAT with a sufficiently high degree of accuracy requires a complicated circuit, which increases the manufacturing cost.
In addition, the technique described in patent document 1 distinguishes between turning and returning of the steering wheel, and changes the gain of the control in accordance with the result of the distinction. Accordingly, there may occur a case in which the vehicle behavior feels unnatural when the vehicle driver turns or returns the steering wheel, because the gain changes at this time. Further, when the vehicle driver intricately operates the steering wheel, it may not be possible to accurately distinguish between the turning and returning of the steering wheel.
As explained above, even according to the technique described in patent document 1, it is intrinsically impossible that the damping control operates only when the steering wheel is returned to improve the vehicle convergence, so that the vehicle driver's operation of the steering wheel is not disturbed.